In recent years, tin oxide has been introduced into the field of semiconductor integrated circuits because of its conductive properties (when appropriately doped) and relative transparency. In particular, tin oxide has been used in fabricating certain charge coupled device (CCD) imagers. In these imagers, tin oxide forms conductive electrodes overlying clocked wells formed in a semiconductor substrate. The electrodes are conventionally formed by depositing a sheet of tin oxide on a gate oxide layer, and then etching the tin oxide sheet to form a plurality of electrodes.
Tin oxide is however a very refractory material and poses special problems in forming microelectronic circuit components with it. Certain wet etches have been reported in the art as successfully etching tin oxide. Among these is an etch using hydrochloric acid and zinc dust. This etch is however insufficiently reliable and does not produce sufficient definition to use in defining tin oxide electrodes in modern-day CCD imagers. Another wet etch known in the art employs hydrogen iodide in an aqueous solution in concentrations ranging from 45 to 51 percent. This etch often produces cracked tin oxide layers and attacks the tin oxide film along its grain boundaries. Chloroform or other carbon-based gases have also not been heretofore satisfactory as plasma reactor etchants because they have tended to deposit a yellow, carbon-based polymer on the reactor apparatus.
A need has therefore arisen in the industry for a tin oxide etching process that can reliably produce tin oxide CCD imagers and other tin oxide semiconductor devices of high quality and resolution.